Why Identical Twins Are Natural Living Laboratories

There is a method in twin research called co-twin control. It happens naturally, experimentally, or by chance when two identical twins have different experiences. In fact, every identical twin pair is a “mini-experiment,” because their environments before and after birth are not exactly the same.

Natural co-twin control happens when identical twin partners show differences in their appearance, health, or behavior that are not caused by any form of intervention. For example, one twin may suffer from depression, take an exciting voyage, or win the lottery. Experimental interventions occur when a researcher subjects one twin to a specific treatment or training program, and provides the other twin with a different experience or no treatment at all. For example, in a classic twin study, one twin may was given training in stair climbing and cube manipulation while the other was not. After some time, the twins’ abilities were compared and found to be similar, suggesting that maturation (and not training) were responsible. Finally, unusual incidents are those in which twins differ due to circumstances beyond their control. For example, one twin may be bullied, while the other is unharmed, or one twin may have been exposed to the measles, while the other was not.

A natural co-twin control study might include the 25 percent of identical twin pairs who show reversals in some characteristics such as hand preference, fingerprint patterns, or direction of the hair whorl. A fascinating case report found that the right-handed twin showed greater left and right hemisphere activity for language and mental rotation tasks, respectively, while her left-handed co-twin showed the reverse pattern.

A wonderful example of an experimental study is the NASA Twins Project. Scott Kelly, an identical twin, spent nearly a year at the International Space Station in 2015-2016, while his brother Mark stayed on Earth. It is unlikely that such a chance will arise again—to see the effects of space travel in such a genetically controlled situation. While Scott and Mark are the first set of identical twin astronauts, neither is the first twin to travel to space. Another identical twin, Charlie Duke, walked on the Moon in 1972 as part of the Apollo 16 mission. However, his brother, Bill, had a congenital heart defect so he could not withstand the rigorous physical activity required of an astronaut.

The most provocative finding from the study of the Kelly twins was the lengthening of Scott’s telomeres, which are the protective caps at the end of the chromosomes. Shortened telomeres were expected because this change is associated with aging, but his telomeres showed the opposite effect; however, his average telomeric length decreased after he returned to Earth, possibly reflecting loss or shortening. Scott’s mental ability also seemed to be affected, but mostly after the flight. His performance speed fell for nearly all ability tests, while his accuracy decreased in all areas except spatial orientation. This decline lasted for half a year after he returned to Earth.

A co-twin control study based on an unusual incident involved 10-year-old identical male twins, one of whom was involved in a near-drowning accident. When they were nearly two-and-a-half, one twin nearly drowned in cold lake water. There was interest in seeing if there were any intellectual deficits in the twin who nearly drowned. The twins then completed a number of ability tests that tapped into different skills. However, meaningful differences between the two brothers were not found. The only exception was that the near-drowning victim scored relatively poorly on one test that suggested attentional problems. Thus, it seems that recovery from a near-drowning accident is possible, but depends on various factors, such as the subject’s age (younger victims are more likely to recover than older ones), water temperature (below 68 degrees Fahrenheit or 20 degrees Celsius is more conducive to recovery than higher temperatures) and submersion time (shorter time underwater generally improves survival chances).

A full version of this article can be found in the journal Twin Research and Human Genetics, 2019.